microheat exchanger for laser diode cooling

ABSTRACT

A microheat exchanging assembly is configured to cool one or more heat generating devices, such as integrated circuits or laser diodes. The microheat exchanging assembly includes a first ceramic assembly thermally coupled to a first surface, and in cases, a second ceramic assembly thermally coupled to a second surface. The ceramic assembly includes one or more electrically and thermally conductive pads to be thermally coupled to a heat generating device, each conductive pad is electrically isolated from each other. The ceramic assembly includes a ceramic layer to provide this electrical isolation. A top surface and a bottom surface of the ceramic layer are each bonded to a conductive layer, such as copper, using an intermediate joining material. A brazing process is performed to bond the ceramic layer to the conductive layer via a joining layer. The joining layer is a composite of the joining material, the ceramic layer, and the conductive layer.

RELATED APPLICATIONS

This application claims priority of U.S. provisional application, Ser.No. 61/188,078, filed Aug. 5, 2008, and entitled “Fabrication ofMicroheat Exchanger for Laser Diode Cooling”, by these same inventors.This application incorporates U.S. provisional application, Ser. No.61/188,078 in its entirety by reference.

FIELD OF THE INVENTION

The invention relates to a microheat exchanger and a method offabrication of the same. More particularly, this invention relates to amicroheat exchanger and a method of fabrication, where the microheatexchanger is used for laser diode cooling.

BACKGROUND OF THE INVENTION

Microheat exchangers are made of thermally conductive material and areused to transfer heat from a heat generating device, such as anintegrated circuit or a laser diode, to a fluid flowing through fluidpathways within the microheat exchanger. Microheat exchangers arecommonly made of metal, such as copper, and electrical isolation isoften required between the heat generating device and the microheatexchanger. Some ceramic materials are thermally conductive yetelectrically resistant. For this reason, such ceramic materials areoften used as an intermediate material between a heat generating deviceand a microheat exchanger to provide electrical isolation while stillmaintaining thermal conductivity. However, it is not practical toconnect a heat generating device directly to ceramic. Instead, the heatgenerating device is coupled to a conductive pad, typically made of aconductive metal such as copper. In such a configuration, the ceramic ismiddle layer between the conductive copper pad coupled to the heatgenerating device and the microheat exchanger.

In order to provide efficient heat transfer from the heat generatingdevice to the microheat exchanger, a good thermal interface betweenceramic and copper is necessary. A direct bonded copper (DBC) methoduses a high temperature joining process to bond a copper sheet to aceramic plate in the presence of a protective gas atmosphere havingsmall amounts of oxygen (50-200 ppm). Exemplary DBC methods aredescribed in U.S. Pat. No. 6,297,469 and U.S. Pat. No. 7,036,711, whichare hereby incorporated in their entirety by reference. Three commonlyused ceramic materials are beryllium oxide (BeO), aluminum oxide(Al₂O₃), and aluminum nitride (AlN). Oxygen and copper bond togetherunder high temperature. The copper and ceramic are heated to a carefullycontrolled temperature, in an atmosphere of nitrogen and a smallpercentage of oxygen. The temperature used is in the range between 1950and 1981 degrees Fahrenheit, which is close to the melting temperatureof copper. Under these conditions, a copper-oxygen eutectic forms whichbonds successfully both to copper and the ceramic, thereby bonding acopper layer to a ceramic layer. The copper layer is used as aconductive pad to be coupled to a heat generating device. The ceramiclayer is typically soldered to the top of the microheat exchanger.

Many problems exist with bonding in general and the DBC technique inparticular. First, application of high temperature to rigid ceramicplates often results in cracking of the ceramic. Second, microvoids areformed at the interface of the bonded copper and ceramic layers. Themicrovoids are due to the imperfections and irregularities in thecontact surfaces of the copper and ceramic layers. For applicationswhere a large heat generating device, or multiple heat generatingdevices are coupled to a single ceramic plate, the size of the ceramicplate is larger. However, the larger the ceramic plate, the greater theimpact of the microvoids. Presence of microvoids reduces thermalefficiency. Further, presence of microvoids increases the chances thatthe copper layer and the ceramic layer will delaminate because there isnot a perfect bond across the entire interface surface.

Third, the thermal coefficient of expansion for copper is much greaterthan that for ceramic. During the high temperature DBC process, thecopper layer expands more so than the ceramic, at which point theceramic layer and the copper layer are bonded. However, upon cooling thecopper layer contracts more so than the ceramic, due to the differingthermal coefficients of expansion, which leads to warping and possiblecracking of the bonded copper-ceramic assembly.

SUMMARY OF THE INVENTION

A microheat exchanging assembly is configured to cool one or more heatgenerating devices, such as integrated circuits or laser diodes. In someembodiments, the microheat exchanging assembly includes a first ceramicassembly thermally coupled to a first surface, and in some embodiments,a second ceramic assembly thermally coupled to a second surface. Eachceramic assembly includes one or more electrically and thermallyconductive pads, each conductive pad is electrically isolated from eachother. Each ceramic assembly includes a ceramic layer to provide thiselectrical isolation. The ceramic layer has high thermal conductivityand high electrical resistivity. A top surface and a bottom surface ofthe ceramic layer are each bonded to a conductive layer, such as copper,using an intermediate joining material. A brazing process is performedto bond the ceramic layer to the conductive layer via a joining layer.The joining layer is a composite of the joining material, the ceramiclayer, and the conductive layer. The top conductive layer and thejoining layer are etched to form the electrically isolated conductivepads. The conductive layers are bonded to the ceramic layer using a bareceramic approach or a metallized ceramic approach.

In one aspect, a device includes a heat exchanging device having athermally conductive material, wherein the heat exchanging device isconfigured to transfer heat from the thermally conductive material to afluid flowing therethrough; and a thermally conductive ceramic assemblythermally coupled to the heat exchanging device. The ceramic assemblyincludes a conductive layer; a ceramic layer; and an active brazingalloy bonded between the conductive layer and the ceramic layer to forma joining layer, wherein the conductive layer and the joining layer areconfigured to form one or more electrically isolated conductive pads. Insome embodiments, the conductive layer and the joining layer arepatterned to form a plurality of electrically isolated pads, furtherwherein each of the plurality of electrically isolated pads areelectrically isolated from each other by the ceramic layer. In someembodiments, the ceramic assembly also includes a second conducive layerand a second active brazing alloy layer bonded between the secondconductive layer and the ceramic layer to form a second joining layer.The device can also include a metal-to-metal joining layer bondedbetween the second conductive layer of the ceramic assembly and the heatexchanging device. In some embodiments, the conductive layer and theheat exchanging device are copper-based. In some embodiments, theceramic layer includes beryllium oxide, aluminum oxide, or aluminumnitride. In some embodiments, the active brazing alloy is a copper-basedactive brazing alloy, a copper-silver-based active brazing alloy, or anindium-copper-silver-based active brazing alloy. In some embodiments,the active brazing alloy layer is an active joining material paste or anactive joining material foil. The device can also include a secondthermally conductive ceramic assembly thermally coupled to an oppositeside of the heat exchanging device than the ceramic assembly.

In another aspect, a device includes a heat exchanging device comprisinga thermally conductive material, wherein the heat exchanging device isconfigured to transfer heat from the thermally conductive material to afluid flowing therethrough; and a thermally conductive ceramic assemblythermally coupled to the heat exchanging device. The ceramic assemblyincludes a conductive layer; a ceramic layer including a metallizedfirst surface; and a joining material bonded between the conductivelayer and the metallized first surface of the ceramic layer to form ajoining layer, wherein the conductive layer, the joining layer, and themetallized first surface are configured to form one or more electricallyisolated conductive pads. In some embodiments, the conductive layer andthe joining layer are patterned to form a plurality of electricallyisolated pads, further wherein each of the plurality of electricallyisolated pads are electrically isolated from each other by the ceramiclayer. The ceramic layer can also include a metallized second surface,and the ceramic assembly can also include a second conducive layer and asecond joining material bonded between the second conductive layer andthe metallized second surface of the ceramic layer to form a secondjoining layer. The device can also include a metal-to-metal joininglayer bonded between the second conductive layer of the ceramic assemblyand the heat exchanging device. In some embodiments, the conductivelayer and the heat exchanging device are copper-based. In someembodiments, the ceramic layer is beryllium oxide, aluminum oxide, oraluminum nitride. In some embodiments, the metallized first surfaceincludes molybdenum manganese and nickel. The device can also include asecond thermally conductive ceramic assembly thermally coupled to anopposite side of the heat exchanging device than the ceramic assembly.In some embodiments, the joining material is a copper-silver paste, acopper-gold paste, a copper-silver foil, or a copper-gold foil. In otherembodiments, the joining material and the conductive layer are a silverplated copper sheet.

In yet another aspect, a device includes a heat exchanging devicecomprising a thermally conductive material, wherein the heat exchangingdevice is configured to transfer heat from the thermally conductivematerial to a fluid flowing therethrough; and a thermally conductiveceramic assembly thermally coupled to the heat exchanging device. Theceramic assembly includes a ceramic layer including a metallized firstsurface; and a conductive layer plated to the metallized first surface,wherein the conductive layer and the metallized first surface areconfigured to form one or more electrically isolated conductive pads. Insome embodiments, the conductive layer and the metallized first surfaceare patterned to form a plurality of electrically isolated pads, furtherwherein each of the plurality of electrically isolated pads areelectrically isolated from each other by the ceramic layer. The ceramiclayer can also include a metallized second surface, and the ceramicassembly can also include a second conductive layer plated to themetallized second surface. The device can also include a metal-to-metaljoining layer bonded between the second conductive layer of the ceramicassembly and the heat exchanging device. In some embodiments, theconductive layer and the heat exchanging device are copper-based. Insome embodiments, the ceramic layer is beryllium oxide, aluminum oxide,or aluminum nitride. In some embodiments, the metallized first surfaceincludes molybdenum manganese and nickel. The device can also include asecond thermally conductive ceramic assembly thermally coupled to anopposite side of the heat exchanging device than the ceramic assembly.

Other features and advantages of the microheat exchanging assembly willbecome apparent after reviewing the detailed description of theembodiments set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cut-out side view of exemplary layers of a ceramicassembly fabricated using a bare ceramic approach according to a firstembodiment, the view shown in FIG. 1 is before a brazing process isperformed.

FIG. 2 illustrates a cut-out side view of exemplary layers of a ceramicassembly fabricated using a bare ceramic approach according to a secondembodiment, the view shown in FIG. 1 is before a brazing process isperformed.

FIG. 3 illustrates an exemplary process for fabricating a ceramicassembly according to the bare ceramic approach.

FIG. 4 illustrates a cut-out side view of exemplary layers of a ceramicassembly fabricated using a brazed copper option of a metallized ceramicapproach according to a first embodiment, the view shown in FIG. 4 isbefore a brazing process is performed.

FIG. 5 illustrates a cut-out side view of exemplary layers of a ceramicassembly fabricated using a metallized ceramic approach according to asecond embodiment, the view shown in FIG. 5 is before a brazing processis performed.

FIG. 6 illustrates an exemplary process for fabricating a ceramicassembly according to the brazed copper option of the metallized ceramicapproach.

FIG. 7 illustrates a cut-out side view of exemplary layers of a ceramicassembly fabricated using a plated copper option of a metallized ceramicapproach according to a third embodiment.

FIG. 8 illustrates an exemplary process for fabricating a ceramicassembly according to the plated copper option of the metallized ceramicapproach.

FIG. 9 illustrates a cut out side view of an exemplary ceramic assembly.

FIGS. 10-11 illustrate two step etching process applied to the exemplaryceramic assembly of FIG. 9.

FIG. 12 illustrates a magnified portion of the etched surfaces betweentwo adjacent pads.

FIG. 13 illustrates the two step etching process applied to theexemplary ceramic assembly of FIG. 9.

FIGS. 14-17 illustrate the second approach for patterning both thecopper layer and the joining layer while fabricating an exemplaryceramic assembly.

FIG. 18 illustrates an exemplary process for fabricating a microheatexchanging assembly according to an embodiment.

FIG. 19 illustrates a cut-out side view of exemplary layers of acompleted microheat exchanging assembly with the ceramic assembliesfabricated using the bare ceramic approach.

FIG. 20 illustrates a cut-out side view of exemplary layers of acompleted microheat exchanging assembly with the ceramic assembliesfabricated using the brazed copper option of the metallized ceramicapproach.

FIG. 21 illustrates a cut-out side view of exemplary layers of acompleted microheat exchanging assembly with the ceramic assembliesfabricated using the plated copper option of the metallized ceramicapproach.

The microheat exchanging assembly is described relative to the severalviews of the drawings. Where appropriate and only where identicalelements are disclosed and shown in more than one drawing, the samereference numeral will be used to represent such identical elements.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Reference will now be made in detail to the embodiments of the microheatexchanging assembly, examples of which are illustrated in theaccompanying drawings. While the microheat exchanging assembly will bedescribed in conjunction with the embodiments below, it will beunderstood that they are not intended to limit the microheat exchangingassembly to these embodiments and examples. On the contrary, themicroheat exchanging assembly is intended to cover alternatives,modifications and equivalents, which may be included within the spiritand scope of the microheat exchanging assembly as defined by theappended claims. Furthermore, in the following detailed description ofthe microheat exchanging assembly, numerous specific details are setforth in order to more fully illustrate the microheat exchangingassembly. However, it will be apparent to one of ordinary skill in theprior art that the microheat exchanging assembly may be practicedwithout these specific details. In other instances, well-known methodsand procedures, components and processes have not been described indetail so as not to unnecessarily obscure aspects of the microheatexchanging assembly.

Embodiments are directed to a microheat exchanging assembly and aceramic assembly and methods of fabricating each. The microheatexchanging assembly is configured to cool one or more heat generatingdevices, such as electronic devices. In some embodiments, the microheatexchanging assembly includes a plurality of electrically and thermallyconductive pads, each conductive pad is electrically isolated from eachother. The heat generating device is electrically and thermally coupledto the conductive pad using any conventional method, such as soldering.In an exemplary application, each pad is coupled to one of an array oflaser diodes used in high power lasers for industrial cutting andmarking applications. In such an application, the microheat exchangingassembly is referred to a microheat exchanger for laser diodes (MELD™).The microheat exchanging assembly is particularly applicable to thoseapplications requiring the arrangement of multiple heat generatingdevices in a common plane, such as a laser diode array. By electricallyisolating each conductive pad, the heat generating devices coupled tothe conductive pads are electrically isolated from each other whilemaintaining a uniform high rate of heat transfer from each heatgenerating device to a microheat exchanger. To provide this electricalisolation a ceramic layer with high thermal conductivity and highelectrical resistivity is used. In some embodiments, the ceramic layeris made of beryllium oxide, aluminum oxide, or aluminum nitride. A topsurface and a bottom surface of the ceramic layer are each bonded to aconductive layer using an intermediate joining material. Brazing of thejoining material during the bonding process enables the liquidus joiningmaterial to melt, which provides a localized “flow” of material into themicrovoids on the contact surfaces of the ceramic and conductive layers,thereby improving thermal efficiency. In some embodiments, eachconductive layer is copper. The top conductive layer and theintermediate joining material are etched to form the electricallyisolated conductive pads. The bonded ceramic and conductive layers forma first ceramic assembly.

The bottom conductive layer of the first ceramic assembly is bonded to atop surface of a microheat exchanger through which a cooling fluidcirculates. The microheat exchanger is made of a thermally conductivematerial. In some embodiments, the microheat exchanger is made ofcopper. Heat is transferred from the heat generating devices coupled tothe conductive pads to the fluid flowing through the microheatexchanger.

In some embodiments, a second ceramic assembly is formed and bonded to abottom surface of the microheat exchanger. The second ceramic assemblycan also include a plurality of electrically isolated conductive pads,which can be patterned the same or differently than those on the firstceramic assembly.

Fabrication of the microheat exchanging assembly includes the generalsteps of fabricating the ceramic assembly, fabricating the microheatexchanger, and final assembly and brazing of the microheat exchangingassembly.

A. Fabrication of the Ceramic Assembly

A ceramic assembly is formed by bonding a conductive layer to both sidesof a thin ceramic plate using an intermediate joining material. In someembodiments, each conductive layer is a copper layer. In someembodiments, the ceramic plate is made of beryllium oxide (BeO),aluminum oxide (Al₂O₃), or aluminum nitride (AlN). The use of BeO mayfind restrictions due to its toxicity. Optimum thickness of the ceramicplate is dictated by the ability to minimize heat transfer resistancewhile maintaining mechanical strength of the bonded layer. The heattransfer resistance is reduced as the thickness of the ceramic plate isreduced, but the mechanical strength is increased as the thickness ofthe ceramic plate is increased. In some embodiments, the ceramic platethickness varies from about 100 micrometers to several millimeters. Instill other embodiments, the thickness of the ceramic plate is in therange of about 0.5 mm to about 0.75 mm. The thickness of each copperlayer is dictated by the extent of warping of the assembled unit and theneed for grinding the copper layer for its planarization. In someembodiments, the copper layer thickness is in the range of about 0.05 mmto about 0.5 mm. In still other embodiments, the thickness of the copperlayer is about 0.25 mm. In some embodiments, the surface area of theceramic assembly is in the range of about 1250 mm² to about 8000 mm².

A requirement of the fabrication of the ceramic assembly is to provideexcellent bonding of the copper layers to the ceramic plate. Other thanthe ceramic plate, the remaining layers of the ceramic assembly are ableto be patterned by selective removal of the copper layer and joiningmaterials for making electrically isolated patterned copper pads.Various techniques for bonding copper to both sides of a ceramic plateare disclosed. One approach is the bare ceramic approach which usesactive brazing alloy (ABA) materials such as copper-based ABA (Cu-ABA);copper and silver-based ABA (CuSil-ABA); and indium, copper, andsilver-based ABA (InCuSil-ABA). Each of these ABAs is copper rich, andtherefore provides good thermal conductivity. Each of these ABAs includea small amount of an active ingredient to bind with ceramic. In someembodiments, each of these ABAs includes titanium (Ti) as an activeingredient. Titanium in the ABA reacts with the ceramic plate and thecopper layer to provide a chemical bond, resulting in a joining layerinterface formed between the copper layer and the ceramic plate. It isunderstood that alternative ABAs including one or more activeingredients other than titanium can be used to bind with ceramic.

Further, the use of a brazing material as the intermediate joiningmaterial provides material “flow” into the microvoids of the contactsurfaces. Brazing is a joining process whereby a joining material, suchas a metal or alloy, is heated to a melting temperature. At the meltingtemperature, the liquidus joining material interacts with a thin layerof the base metal, cooling to form a strong, sealed joint. The resultingjoining layer is a blend of the ABA material, the copper layer, and theceramic layer. The melting temperature of the braze material is lowerthan the melting temperature of the materials being joined. Using thebrazing process to bond the ceramic layer to the copper layer, thebrazing temperature is lower than a conventional temperature used todirect copper bond the two layers together. Reducing the temperaturealso reduces the warping effects on the cooled copper-ceramic assembly.

Table 1 shows the composition and melting temperature of some selectedactive brazing alloys used in the copper and ceramic bonding process:

TABLE 1 Active Brazing Solidus Liquidus Alloy Composition Temperature, °F. Temperature, ° F. Cu-ABA Cu - 92.75% 1756 1875 Ti - 2.25% Al - 2.00%Si - 3.00% CuSil-ABA Ag - 63.00% 1435 1500 Cu - 32.25% Ti - 1.25%InCuSil-ABA Ag - 59.00% 1121 1319 Cu - 27.25% In - 12.50% Ti - 1.25%In some embodiments, each copper layer is a copper sheet and the ABA isused in a paste form. The ABA paste can be sprayed or screen printed oneither both sides of the ceramic plate or on one side of each coppersheet that is to be attached to the ceramic plate. FIG. 1 illustrates acut-out side view of exemplary layers of a ceramic assembly 10fabricated using a bare ceramic approach according to a firstembodiment, the view shown in FIG. 1 is before a brazing process isperformed. The first embodiment of the bare ceramic approach uses an ABApaste. The ABA paste can be applied either to one side of each coppersheet, as shown in FIG. 1 as ABA paste 14 applied to copper sheet 12 andABA paste 18 applied to copper sheet 20, or to both sides of a ceramicplate 16. In some embodiments, the thickness of the ABA paste variesbetween several microns to 100s of microns. In still other embodiments,the thickness of the ABA paste is about 25 microns.

In other embodiments, an ABA foil is placed between the ceramic plateand each copper sheet. FIG. 2 illustrates a cut-out side view ofexemplary layers of a ceramic assembly 30 fabricated using a bareceramic approach according to a second embodiment, the view shown inFIG. 1 is before a brazing process is performed. The second embodimentof the bare ceramic approach uses ABA foils. A first ABA foil 34 ispositioned between one side of a ceramic plate 36 and a copper sheet 32,and a second ABA foil 38 is positioned between the other side of theceramic plate 36 and a copper sheet 40. In some embodiments, thethickness of each ABA foil 34 and 38 is in the range of about 10 micronsto about 100 microns. In still other embodiments, the thickness of eachABA foil 34 and 38 is about 25 microns.

FIG. 3 illustrates an exemplary process for fabricating a ceramicassembly according to the bare ceramic approach. At the step 22, thecopper layer and ABA material are assembled on both sides of the ceramicplate. The ABA material can be either paste of a foil. At the step 24,the copper, ABA material, ceramic plate, ABA material, and copperassembly are vacuum brazed, thereby forming the ceramic assembly. Sincethe ABAs described in this bonding process contain titanium, the use offorming gas (95% Nitrogen/5% Hydrogen) must be avoided since using suchgas in the brazing process with these alloys forms titanium hydride,which prevents chemical bonding of ceramic to copper. The copper layeris made of any conventional copper alloy including, but not limited to,110, 102, or 101 copper. When the ABA material is Cu-ABA, the brazingtemperature is in the range of about 1840 to about 1890 degreesFahrenheit. When the ABA material is CuSil-ABA, the brazing temperatureis in the range of about 1460 to about 1520 degrees Fahrenheit. When theABA material is InCuSil-ABA, the brazing temperature is in the range ofabout 1280 to about 1340 degrees Fahrenheit.

Another approach for bonding copper to both sides of a ceramic plate isthe metallized ceramic approach which uses a high temperature refractorymaterial including, but not limited to, molybdenum manganese (MoMn),titanium (Ti), or tungsten (W). In some embodiments, the refractorymaterials, such as MoMn paste, are screen printed onto each side of aceramic plate. In other embodiments, the refractory materials, such astitanium or tungsten, are deposited by physical vapor deposition (PVD)onto a first side and a second side of a ceramic plate. The next step ofmetallization is to provide a thin layer coating of electrolytically orelectrolessly deposited nickel, thereby forming a metallized ceramicplate. The nickel layer enables joining of the metallized ceramic plateto copper, or electroplating of copper directly onto the metallizedceramic plate. The metallized ceramic approach includes at least twooptions for bonding the metallized ceramic plate to copper.

A first option is the brazed copper option where a copper sheet isbrazed to both sides of the metallized ceramic plate. In someembodiments, each copper sheet is plated with a thin layer of eithersilver or gold, which reacts with copper to form CuSil or CuAu,respectively, during bonding. FIG. 4 illustrates a cut-out side view ofexemplary layers of a ceramic assembly 50 fabricated using a brazedcopper option of a metallized ceramic approach according to a firstembodiment, the view shown in FIG. 4 is before a brazing process isperformed. The first embodiment of the metallized ceramic approach usesa metallized ceramic plate and plated copper sheets. As shown in FIG. 4,a ceramic plate 54 includes metallized layers 56 and 58. Copper sheet 51is plated by a layer 52. Copper sheet 60 is plated by a layer 62. Insome embodiments, the plated layers 52 and 62 each have a thicknessbetween about 1 micron and about 100 microns. In still otherembodiments, the plated layers 52 and 62 each have a thickness of about10 microns.

In other embodiments, a thin sheet of brazing alloy is placed betweenthe metallized ceramic plate and each copper sheet. Examples of brazingalloy sheets include, but are not limited to, copper-silver-based sheets(CuSil sheets) or copper-gold-based sheets (CuAu sheets). FIG. 5illustrates a cut-out side view of exemplary layers of a ceramicassembly 70 fabricated using a metallized ceramic approach according toa second embodiment, the view shown in FIG. 5 is before a brazingprocess is performed. The second embodiment of the metallized ceramicapproach uses brazing alloy sheets. A first brazing alloy sheet 74 ispositioned between a metallized layer 78 of a ceramic plate 76 and acopper sheet 72. A second brazing alloy sheet 82 is positioned between ametallized layer 80 of the ceramic plate 76 and a copper sheet 84. Insome embodiments, the thickness of each brazing alloy sheet 74 and 82 isin the range of about 10 microns to about 100 microns. In still otherembodiments, the thickness of each brazing alloy sheet 74 and 82 isabout 25 microns.

FIG. 6 illustrates an exemplary process for fabricating a ceramicassembly according to the brazed copper option of the metallized ceramicapproach. At the step 140, a metallized layer is applied to a topsurface and a bottom surface of a ceramic plate. An exemplary processfor applying the metallized layer includes applying a high temperaturerefractory material ink to the ceramic plate, and firing the ceramicplate and refractory material ink, electroplating a layer of Ni onto therefractory material layer, firing the ceramic plate, refractory materiallayer, and the Ni plating to form the metallized ceramic plate. Therefractory material ink can be applied by screen printing or deposition.In some embodiments, the refractory material layer has a thickness inthe range of about 10 microns to about 20 microns, the ceramic plate andrefractory material ink are fired at a temperature of 2515 degreesFahrenheit, the Ni layer has a thickness of about 2 microns, and theceramic plate, refractory material layer, and Ni plating are fired at atemperature of about 1380 degrees Fahrenheit.

At the step 142, a brazing material is placed between the metallizedceramic layer and each of two copper sheets. In some embodiments, thebrazing material is either silver or gold which is plated onto eachcopper sheet. In other embodiments, the brazing material is a brazingalloy sheet, such as a CuSil sheet or a CuAu sheet, which is positionedbetween the metallized ceramic plate and each of the copper sheets. Ineither case, each copper sheet is made of any conventional copper alloyincluding, but not limited to, 110, 102, or 101 copper. At a step 144,the copper sheet and brazing material are assembled on both sides of themetallized ceramic plate. At the step 146, the assembly from step 144 isvacuum brazed, thereby forming the ceramic assembly. In someembodiments, the brazing temperature of the step 146 is about 1510degrees Fahrenheit.

A second option of the metallized ceramic approach for bonding themetallized ceramic plate to copper is the plated copper option whichelectroplates copper onto both sides of the metallized ceramic plate.FIG. 7 illustrates a cut-out side view of exemplary layers of a ceramicassembly 90 fabricated using a plated copper option of a metallizedceramic approach according to a third embodiment. The third embodimentof the metallized ceramic approach uses a metallized ceramic plate whichis then plated with copper. As shown in FIG. 7, a ceramic plate 94includes metallized layers 96 and 98. Copper is plated on top of themetallized layers 96 and 98 to form plated copper layers 92 and 100.

FIG. 8 illustrates an exemplary process for fabricating a ceramicassembly according to the plated copper option of the metallized ceramicapproach. At the step 150, a metallized layer is applied to a topsurface and a bottom surface of a ceramic plate, thereby forming ametallized ceramic plate. The step 150 is performed in a similar manneras the step 140 of FIG. 6. To enhance adhesion of plated copper, at thestep 152, the outer surface on the metallized ceramic plate undergoes acleaning step to remove the oxide layers. This cleaning step enhancesadhesion of plated copper. At the step 154, copper is plated onto eachmetallized layer of the metallized ceramic plate. Plating installationand fixtures are configured to take into consideration the currentdistribution aspects to provide uniform deposition of up to 300 micronthick copper layers on both sides of the metallized ceramic layer.

The ceramic assemblies formed by the above methods include a joininglayer formed by bonding an intermediate joining material between eachcopper layer and the ceramic layer. The intermediate joining materialprovides a strong bonding of copper to the ceramic plate. Theintermediate joining material is shown FIGS. 1, 2, 4, 5, and 7 as adiscrete layer distinct from the adjoining copper and ceramic layers,this is the condition prior to the brazing process that bonds the copperto the ceramic. Once the brazing process is completed, the intermediatejoining material and the surfaces of the adjoining copper and ceramiclayers diffuse together to form a mixed interface material, referred toas a joining layer. It is understood that any reference to the bondedjoining material after the brazing process is performed is intended torepresent the joining layer.

The ceramic assembly described above provides a single device interfacesurface to which a heat generating device can be coupled. In someapplications, multiple heat generating devices are to be coupled to theceramic assembly. To accommodate multiple heat generating devices, alarger sized ceramic plate is used. In some embodiments, the width ofthe ceramic plate is about 50 mm and the length of the ceramic plate isabout 160 mm. However, if the heat generating devices are coupled to theceramic assembly with the single device interface surface, there is notelectrical isolation between each of the coupled heat generatingdevices. Therefore, to provide electrical isolation between each of themultiple heat generating devices, electrically isolated copper pads areformed on one side of the ceramic assembly. To electrically isolate eachpad, both the copper layer and the joining layer are etched to theceramic layer. It is necessary to completely etch down to the ceramiclayer to provide electrical isolation for each pad. If any joiningmaterial remains to connect the pads, electrical isolation is notachieved as the joining layer is electrically conductive.

Photopatterning includes selective removal of material through patternedphotoresist and can be accomplished by wet etching or a combination ofwet etching and physical methods of material removal, such as laseretching or bead blasting. The copper layer can be easily photopatternedusing any conventional wet etch process. However, the joining layer isdifficult to photopattern by wet etching. The joining layer can be wetetched but at the expense of over-etching the copper layer becausecopper is etched at a greater rate than the joining layer. A number ofapproaches are disclosed to pattern both the copper layer and thejoining layer formed between the copper layer and the ceramic layer. Thefirst approach uses a physical etch step. The physical etch step is anyconventional physical method for removing material including, but notlimited to, laser etching and bead blasting. The physical etch step isused either as part of a two step etching process or a single stepetching process. In the two step etching process, a first wet etch stepis performed to selectively etch the outer copper layer. A secondphysical etch step is then performed on the joining layer at the pointsexposed by the preceding wet etch performed on the copper layer. In thesingle etch step, a physical etch step is performed to simultaneouslyetch both the copper layer and the joining layer.

FIG. 9 illustrates a cut out side view of an exemplary ceramic assembly110. FIGS. 10-11 illustrate two step etching process applied to theexemplary ceramic assembly 110. As shown in FIG. 9, a joining layer 114is formed between a ceramic layer 116 and a copper layer 112, and ajoining layer 118 is formed between the ceramic layer 116 and a copperlayer 120. FIG. 10 shows a patterned copper layer 112′ after anexemplary selective wet etch process is performed. The slopes 122 of theetched copper walls are exaggerated to indicate the effects of the wetetch process. FIG. 11 shows a patterned joining layer 114′ after aphysical etch process is performed on the portions of the joining layer114 exposed after the wet etch process. The patterned copper layer 112′and the patterned joining layer 114′ form electrically isolated pads130. The number and dimensions of the pads 130 shown in FIG. 11 is forexemplary purposes only. The slopes 124 of the etched joining layerwalls are steeper than the slopes 122 of the etched copper walls. Thedifferent slopes are an artifact of the two different etch processes.FIG. 12 illustrates a magnified portion of the etched surfaces betweentwo adjacent pads 130 to better illustrate the difference in the slope122 of the etched copper wall and the slope 124 of the etched joininglayer wall. The slopes 122 and 124 shown in FIGS. 9-12 are for exemplarypurposes only.

FIG. 13 illustrates the two step etching process applied to theexemplary ceramic assembly 110 of FIG. 9. FIG. 13 shows a patternedcopper layer 112″ and a patterned joining layer 114″ after an exemplaryselective physical etch process is simultaneously performed on bothlayers. The patterned copper layer 112″ and the patterned joining layer114″ form electrically isolated pads 130′. The number and dimensions ofthe pads 130′ shown in FIG. 13 is for exemplary purposes only. Theslopes 122′ of the etched copper walls and the slopes 124′ of the etchedjoining layer walls are the same as both are formed using the singlestep physical etch process.

A second approach for patterning both the copper layer and the joininglayer uses patterned screen printings to selectively apply theintermediate joining material on each side of the ceramic layer. FIGS.14-17 illustrate the second approach for patterning both the copperlayer and the joining layer while fabricating an exemplary ceramicassembly 160. In FIG. 14, a screen printed pattern of intermediatejoining material 164 is applied to a first surface of a ceramic plate166, and a screen printed pattern of intermediate joining material 168is applied to a second surface of the ceramic plate 166. Theintermediate joining material 164 and 168 is either an ink or paste toenable the screen printing application. In some embodiments, thethickness of the intermediate joining material is between severalmicrons to 100s of microns. In still other embodiments, the thickness ofthe intermediate joining material is about 25 microns. In alternativeconfigurations, the intermediate joining layer 168 is not patterned,similarly to the ceramic assembly 110 in FIGS. 9-13. In general, aceramic assembly can be patterned on one or both sides depending on theapplication.

In FIG. 15, a copper sheet 162 is positioned against the patternedintermediate joining material 164, and a copper sheet 170 is positionedagainst the patterned intermediate joining material 168. The assembly isthen brazed. The brazing temperature is determined by the type ofintermediate joining material used, such as described in Table 1.

In FIG. 16, a photoresist layer 172 is applied and patterned on thecopper sheet 162, and a photoresist layer 174 is applied and patternedon the copper sheet 170. The photoresist layer 172 is patterned to matchthe patterned joining material 164, and the photoresist layer 174 ispatterned to match the patterned joining material 168.

In FIG. 17, the copper sheets 162 and 170 are selectively etched. Insome embodiments, the copper sheets are etched using a wet etch process.The photoresist layers 172 and 174 are then removed. The result is aplurality of electrically isolated conductive pads 180. The number anddimensions of the pads 180 shown in FIG. 17 is for exemplary purposesonly.

Using either the first approach, shown in FIGS. 9-13, or the secondapproach, shown in FIGS. 14-17, electrical isolation of the conductivepads can be verified by measuring resistivity between the pads. Thefinal step in fabricating the ceramic assembly is laser machining of theassembly to provide drilled alignment holes and final shaping of theassembly.

B. Fabrication of the Microheat Exchanger

The microheat exchanger is made of a thermally conductive material. Insome embodiments, the microheat exchanger is made of copper. Themicroheat exchanger includes fluid pathways that enable fluid flowthrough the microheat exchanger. Heat is transferred from the thermallyconductive material to fluid flowing through the microheat exchanger.The microheat exchanger includes one or more fluid input ports and oneor more fluid output ports to enable fluid flow into and out of themicroheat exchanger. In some embodiments, fluid pathways within themicroheat exchanger are formed from cross hatched patterned fin designto provide flow uniformity either across the entire microheat exchangeror to select portions of the microheat exchanger. When the microheatexchanger is coupled to the ceramic assembly, the fluid pathways aredesigned to provide flow uniformity over the length of each heatgenerating device coupled to the conductive pads on the ceramicassembly. In some embodiments, the patterned fins are brazed to themicroheat exchanger body using a CuSil sheet. In some embodiments, thethickness of the CuSil sheet is in the range of about 10 micrometer toabout 100 micrometers. In still other embodiments, the thickness of theCuSil sheet is about 25 microns. It is understood that any conventionalmicroheat exchanger that includes fluid flow therethrough can be used.

C. Fabrication of the Microheat Exchanging Assembly

Final assembly involves placing and aligning a first ceramic assembly,the microheat exchanger, and a second ceramic assembly in a fixture andbrazing the fixed assembly in a vacuum or forming gas furnace. In someembodiments, only a single ceramic assembly is brazed to the microheatexchanger. A joining material is used to braze each ceramic assembly tothe microheat exchanger. Where the microheat exchanger is made of copperand the bottom conducting layer of the ceramic assembly is also a copperlayer, the joining material is a copper-to-copper joining material. Insome embodiments, the joining material is a CuSil paste or CuSil foil.In an exemplary application, an eutectic CuSil joining material is madeof 72% silver and 28% copper, having a melting temperature of 1435degrees Fahrenheit. Using this CuSil joining material, a brazingtemperature is about 1420 degrees Fahrenheit. Using a brazing processthe joining material “flows” into the microvoids on the contactsurfaces. Also, the brazing temperature and pressure for bonding theceramic assembly to the microheat exchanger is lower than the brazingtemperature and pressure used to fabricate the ceramic assembly. Assuch, using two separate fabrication steps, one to fabricate the ceramicassembly and another to fabricate the microheat exchanging assembly,does not put the microheat exchanger under as high a temperature orpressure, which reduces the chance of deformation. In other embodiments,the joining material is a solder paste or a solder foil. In general, anyconventional metal-to-metal joining material can be used. In someembodiments, the thickness of the joining material is in the range ofabout 10 micrometer to about 100 micrometers. In still otherembodiments, the thickness of the joining material is about 25 microns.In an alternative approach, instead of applying a separate joiningmaterial, the microheat exchanger body is plated with silver which formsCuSil during brazing. In some embodiments, the silver plating thicknessis between about 1 micron and about 100 microns. In still otherembodiments, the thickness of the silver plating is about 10 microns.

FIG. 18 illustrates an exemplary process for fabricating a microheatexchanging assembly according to an embodiment. At the step 190, a firstceramic assembly and a joining material are assembled on a first surfaceof a microheat exchanger. At the step 192, a second ceramic assembly anda joining material are assembled on a second surface of the microheatexchanger. The joining material can be either paste or a foil. In someembodiments, one or more surfaces of the first ceramic assembly arepatterned. In other embodiments, one or more surfaces of both the firstceramic assembly and the second ceramic assembly are patterned. In stillother embodiments, the step 192 is not performed and only a singleceramic assembly and joining material are assembled to the microheatexchanger. At the step 194, the first ceramic assembly, the joiningmaterial, the microheat exchanger, the joining material, and the secondceramic assembly are brazed, thereby forming the microheat exchangingassembly.

FIG. 19 illustrates a cut-out side view of exemplary layers of acompleted microheat exchanging assembly 210 with the ceramic assembliesfabricated using the bare ceramic approach. The microheat exchangingassembly 210 includes a patterned ceramic assembly bonded to a firstsurface of a microheat exchanger 224, and a ceramic assembly bonded to asecond surface of the microheat exchanger 224. The patterned ceramicassembly includes a patterned copper layer 212, a patterned ABA joininglayer 214, a ceramic plate 216, an ABA joining layer 218, and a copperlayer 220. The copper layer 212 and the joining layer 214 are patternedto form electrically isolated conductive pads 238. The copper layer 220is bonded to the microheat exchanger 224 via joining layer 222. Theceramic assembly includes a copper layer 228, an ABA joining layer 230,a ceramic plate 232, an ABA joining layer 234, and a copper layer 236.The copper layer 228 is bonded to the microheat exchanger 224 viajoining layer 226. Although only the copper layer 212 and the joininglayer 214 are shown to be patterned in FIG. 19, it is understood thatthe copper layer 220 and the joining layer 218, the copper layer 228 andthe joining layer 230, and/or the copper layer 236 and the joining layer234 can be patterned according to the application.

As describe above in the bare ceramic approach of fabricating theceramic assembly, the ABA joining material can be applied as a foil or apaste. In some embodiments, the ABA joining material is Cu-ABA,CuSil-ABA, or InCuSil-ABA. In some embodiments, the joining materialused for the joining layers 222 and 226 is a CuSil paste or a CuSilfoil. In other embodiments, the joining material is a solder paste or asolder foil. In general, any conventional metal-to-metal joiningmaterial can be used.

FIG. 20 illustrates a cut-out side view of exemplary layers of acompleted microheat exchanging assembly 310 with the ceramic assembliesfabricated using the brazed copper option of the metallized ceramicapproach. The microheat exchanging assembly 310 includes a patternedceramic assembly bonded to a first surface of a microheat exchanger 328,and a ceramic assembly bonded to a second surface of the microheatexchanger 328. The patterned ceramic assembly includes a patternedcopper layer 312, a patterned joining layer 314, a metallized layer 316,a ceramic plate 318, a metallized layer 320, a joining layer 322, and acopper layer 324. The copper layer 312, the joining layer 314, and themetallized layer 316 are patterned to form electrically isolatedconductive pads 346. The copper layer 324 is bonded to the microheatexchanger 328 via joining layer 326. The ceramic assembly includes acopper layer 332, a joining layer 334, a metallized layer 336, a ceramicplate 338, a metallized layer 340, a joining layer 342, and a copperlayer 344. The copper layer 332 is bonded to the microheat exchanger 328via joining layer 330. Although only the copper layer 312, the joininglayer 314, and the metallized layer 316 are shown to be patterned inFIG. 20, it is understood that the copper layer 324, the joining layer322, and the metallized layer 320, the copper layer 332, the joininglayer 334, and the metallized layer 336, and/or the copper layer 344,the joining layer 342, and the metallized layer 340 can be patternedaccording to the application.

As describe above in the brazed copper option of the metallized ceramicapproach for fabricating the ceramic assembly, the metallized layerincludes refractory materials, such as molybdenum manganese (MoMn),titanium (Ti), or tungsten (W), plated with nickel. The joining materialused to form the joining layers 314, 322, 334, and 342 can be applied asa foil or a paste. In some embodiments, the joining material is a CuSilor CuAu paste or a CuSil or CuAu foil. In other embodiments, the joiningmaterial and copper layer are combined as a silver plated copper sheet.In some embodiments, the joining material used for the joining layers326 and 330 is a CuSil paste or a CuSil foil. In other embodiments, thejoining material is a solder paste or a solder foil. In general, anyconventional metal-to-metal joining material can be used for the joininglayers 326 and 330.

FIG. 21 illustrates a cut-out side view of exemplary layers of acompleted microheat exchanging assembly 410 with the ceramic assembliesfabricated using the plated copper option of the metallized ceramicapproach. The microheat exchanging assembly 410 includes a patternedceramic assembly bonded to a first surface of a microheat exchanger 424,and a ceramic assembly bonded to a second surface of the microheatexchanger 424. The patterned ceramic assembly includes a patternedcopper layer 412, a patterned metallized layer 414, a ceramic plate 416,a metallized layer 418, and a copper layer 420. The copper layer 412 andthe metallized layer 414 are patterned to form electrically isolatedconductive pads 438. The copper layer 420 is bonded to the microheatexchanger 424 via joining layer 422. The ceramic assembly includes acopper layer 428, a metallized layer 430, a ceramic plate 432, ametallized layer 434, and a copper layer 436. The copper layer 428 isbonded to the microheat exchanger 424 via joining layer 426. Althoughonly the copper layer 412 and the metallized layer 414 are shown to bepatterned in FIG. 21, it is understood that the copper layer 420 and themetallized layer 418, the copper layer 428 and the metallized layer 430,and/or the copper layer 436 and the metallized layer 434 can bepatterned according to the application.

As describe above in the plated copper option of the metallized ceramicapproach for fabricating the ceramic assembly, the metallized layerincludes refractory materials, such as molybdenum manganese (MoMn),titanium (Ti), or tungsten (W), plated with nickel. The joining materialcan be applied as a foil or a paste. In some embodiments, the joiningmaterial is a CuSil or CuAu paste or a CuSil or a CuAu foil. In otherembodiments, the joining material is a solder paste or a solder foil. Ingeneral, any conventional metal-to-metal joining material can be usedfor the joining layers 422 and 426.

The microheat exchanging assemblies are described above as bonding anouter surface of the ceramic assembly to an outer surface of themicroheat exchanger via a joining material. In alternative embodiments,an intermediate layer, layers stack, block, or device, such as anadditional microheat exchanger, can be positioned between the ceramicassembly and the microheat exchanger, where the intermediate layer,layers stack, block, or device is thermally conductive and includesouter surfaces conducive for bonding with the outer surface of theceramic assembly and the outer surface of the microheat exchanger asdescribed above.

The microheat exchanger has been described in terms of specificembodiments incorporating details to facilitate the understanding of theprinciples of construction and operation of the microheat exchanger.Such reference herein to specific embodiments and details thereof is notintended to limit the scope of the claims appended hereto. It will beapparent to those skilled in the art that modifications may be made inthe embodiment chosen for illustration without departing from the spiritand scope of the microheat exchanger.

1. A device comprising: a. a heat exchanging device comprising athermally conductive material, wherein the heat exchanging device isconfigured to transfer heat from the thermally conductive material to afluid flowing therethrough; and b. a thermally conductive ceramicassembly thermally coupled to the heat exchanging device, wherein theceramic assembly comprises: i. a conductive layer; ii. a ceramic layer;and iii. an active brazing alloy bonded between the conductive layer andthe ceramic layer to form a joining layer, wherein the conductive layerand the joining layer are configured to form one or more electricallyisolated conductive pads.
 2. The device of claim 1 wherein theconductive layer and the joining layer are patterned to form a pluralityof electrically isolated pads, further wherein each of the plurality ofelectrically isolated pads are electrically isolated from each other bythe ceramic layer.
 3. The device of claim 1 wherein the ceramic assemblyfurther comprises a second conducive layer and a second active brazingalloy layer bonded between the second conductive layer and the ceramiclayer to form a second joining layer.
 4. The device of claim 3 furthercomprising a metal-to-metal joining layer bonded between the secondconductive layer of the ceramic assembly and the heat exchanging device.5. The device of claim 1 wherein the conductive layer and the heatexchanging device are copper-based.
 6. The device of claim 1 wherein theceramic layer comprises beryllium oxide, aluminum oxide, or aluminumnitride.
 7. The device of claim 1 wherein the active brazing alloycomprises a copper-based active brazing alloy, a copper-silver-basedactive brazing alloy, or an indium-copper-silver-based active brazingalloy.
 8. The device of claim 1 wherein the active brazing alloy layeris comprised of an active joining material paste or an active joiningmaterial foil.
 9. The device of claim 1 further comprising a secondthermally conductive ceramic assembly thermally coupled to an oppositeside of the heat exchanging device than the ceramic assembly.
 10. Adevice comprising: a. a heat exchanging device comprising a thermallyconductive material, wherein the heat exchanging device is configured totransfer heat from the thermally conductive material to a fluid flowingtherethrough; and b. a thermally conductive ceramic assembly thermallycoupled to the heat exchanging device, wherein the ceramic assemblycomprises: i. a conductive layer; ii. a ceramic layer including ametallized first surface; and iii. a joining material bonded between theconductive layer and the metallized first surface of the ceramic layerto form a joining layer, wherein the conductive layer, the joininglayer, and the metallized first surface are configured to form one ormore electrically isolated conductive pads.
 11. The device of claim 10wherein the conductive layer and the joining layer are patterned to forma plurality of electrically isolated pads, further wherein each of theplurality of electrically isolated pads are electrically isolated fromeach other by the ceramic layer.
 12. The device of claim 10 wherein theceramic layer further comprises a metallized second surface, and theceramic assembly further comprises a second conducive layer and a secondjoining material bonded between the second conductive layer and themetallized second surface of the ceramic layer to form a second joininglayer.
 13. The device of claim 12 further comprising a metal-to-metaljoining layer bonded between the second conductive layer of the ceramicassembly and the heat exchanging device.
 14. The device of claim 10wherein the conductive layer and the heat exchanging device arecopper-based.
 15. The device of claim 10 wherein the ceramic layercomprises beryllium oxide, aluminum oxide, or aluminum nitride.
 16. Thedevice of claim 10 wherein the metallized first surface comprisesmolybdenum manganese and nickel.
 17. The device of claim 10 furthercomprising a second thermally conductive ceramic assembly thermallycoupled to an opposite side of the heat exchanging device than theceramic assembly.
 18. The device of claim 10 wherein the joiningmaterial comprise a copper-silver paste, a copper-gold paste, acopper-silver foil, or a copper-gold foil.
 19. The device of claim 10wherein the joining material and the conductive layer comprise a silverplated copper sheet.
 20. A device comprising: a. a heat exchangingdevice comprising a thermally conductive material, wherein the heatexchanging device is configured to transfer heat from the thermallyconductive material to a fluid flowing therethrough; and b. a thermallyconductive ceramic assembly thermally coupled to the heat exchangingdevice, wherein the ceramic assembly comprises: i. a ceramic layerincluding a metallized first surface; and ii. a conductive layer platedto the metallized first surface, wherein the conductive layer and themetallized first surface are configured to form one or more electricallyisolated conductive pads.
 21. The device of claim 20 wherein theconductive layer and the metallized first surface are patterned to forma plurality of electrically isolated pads, further wherein each of theplurality of electrically isolated pads are electrically isolated fromeach other by the ceramic layer.
 22. The device of claim 20 wherein theceramic layer further comprises a metallized second surface, and theceramic assembly further comprises a second conductive layer plated tothe metallized second surface.
 23. The device of claim 22 furthercomprising a metal-to-metal joining layer bonded between the secondconductive layer of the ceramic assembly and the heat exchanging device.24. The device of claim 20 wherein the conductive layer and the heatexchanging device are copper-based.
 25. The device of claim 20 whereinthe ceramic layer comprises beryllium oxide, aluminum oxide, or aluminumnitride.
 26. The device of claim 20 wherein the metallized first surfacecomprises molybdenum manganese and nickel.
 27. The device of claim 20further comprising a second thermally conductive ceramic assemblythermally coupled to an opposite side of the heat exchanging device thanthe ceramic assembly.